Antennas



United States atent ANTENNAS Arthur Dorne, Glen Head, N. Y., assignor to Dome and Margolin, Inc., Westbury, N. Y., a corporation of New York Application May 1, 1956, Serial No. 581,909

14 Claims. (Cl. 343-769) This invention relates to antennas for the transmission or reception of radio signals and more particularly relates to improvements on recessed slot antennas of the type described and claimed in Dorne Patent No. 2,644,090, granted June 30, 1953.

As is described in the above mentioned patent, the continued rapid development of higher speed aircraft has ycreated the necessity for the use of radio antennas which do not project outwardly from the body of the aircraft, so as not to offer resistance to the movement of the craft. Concurrently with the development of such aircraft, whose higher operating speeds dictate further requirements for improved safety, there has been the development of improved radio aids for aircraft navigation. Thus, radio ranges link the major cities of the country, for the guidance of aircraft from one point to another. Similarly, omnidirectional radio ranges are located at various points throughout the United States. In addition to these radio services, instrument landing systems, ground lcontrolled approach systems, and other radio services in addition to communication, have become a standard requirement in many types of aircraft.

The necessity of being able to receive such a Wide variety of radio services, each with an increasing number of stations or channels, has required widening the operative band width of the antennas in use. This need is a particularly pressing one in the very high and ultra high frequency ranges of the frequency spectrum, and is supplied by the present invention.

in accordance with the present invention, an improved recessed slot antenna is provided suitable for ush mounting in an aircraft, which combines the advantages of reasonable size at ultra-high and very-high frequency bands, with wide frequency band, permitting the use of a -single antenna for a greater variety of radio services and to receive a greater number of channels or stations.

Accordingly, it is an object of this invention to proa' vide an improved wide-band recessed slot antenna particularly suitable for use on aircraft.

It is another object of this invention to provide a recessed antenna of the slot type incorporating specific features for improving its band Width.

ln particular, it is an object of the invention to provide an annular-slot recessed antenna system having a cooperating parasitic annular slot for improved band width.

lt is still another object of the present invention to provide an improved annular-slot recessed antenna system with a special coupling arrangement for improved band width.

Other objects and advantages of the present invention will become more apparent from consideration of the following detailed description taken in conjunction with the accompanying drawings, in which Fig. 1 is a perspective cross sectional view of one form of antenna according to the present invention.

Fig. 2 is a schematic electrical circuit diagram of the equivalent circuit of the antenna of Fig. l.

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Fig. 3 is a similar perspective cross sectional View of a modified form of antenna according to the present invention and particularly adapted for use in curved surfaces of aircraft.

Referring to Fig. 1, a conventional coaxial line coupling is schematically shown, having an inner conductor 11 and an outer conductor 12. It will be understood that this coupling 11, 12 is adapted to be connected to a coaxial transmission line connected to the radio receiver and/or transmitter apparatus. This coupling 11, 12 is directly connected to a radial conical transmission line section formed by a conical conductive member 13 and a circular conductive plate 14. Conical member 13 is in turn connected to a conductive aring conical member 15 which has a flange 16 parallel to and juxtaposed to the outer edge of circular plate 14 so as to form an annular distributed capacitance therewith. Connected between the ange 16 and the outer edge of plate 14 are a plurality of posts or Wires 17 illustrated in this instance as four in number, and serving as inductance elements. These posts 17 may extend directly from plate 14 to flange 16 or, alternatively, as shown in the drawing, cylindrical recesses 13 having closed bottoms 19 may be formed concentrically with the posts 17 in ange 16 to which the lower ends of the posts 17 are respectively connected. This structure has the advantage of permitting the posts 17 to provide increased inductance where that is desirable, or to assist in resonating the posts 17 and the annular capacitance already referred to. The recesses 1S may be made adjustable in depth, as by making their bottoms 19 in the form of disks threaded into cylinders 1d and posts 17 may then be screws threaded into such disks and into the rim 20 of plate 14. Of course, posts 17 may be conductively connected to rim 20 and flange 16 in any desired manner.

The annular gap 21 between flange 16 and rim 20 is electromagnetically coupled to a cavity 22 of annular conformation bounded by the Haring conical member 15, a flat annular conductive disk 23 forming the bottom of the antenna structure, an outer cylindrical wall 24, an inner cylindrical wall 25 extending upwardly from plate 14 and an annular conductive member 26 connected to cylinder 25 and extending toward cylinder 24 but leaving a gap or slot 27 therebetween. This gap 27 then forms the radiating slot for the antenna structure.

Concentrically within the annular cavity 22 is a structure defining a second annular cavity 28 having an outer cylindrical side wall formed by cylinder 25, a bottom wall formed by plate 14, an inner wall formed by a further cylindrical member 29, and a top wall formed by the outer portion or rim of a disk 31 of circular shape. Disk 31 extends only part way between cylindrical walls 29 and 25 leaving a gap or slot 32 therebetween which electromagnetically couples the cavity 28 to the surrounding external space and to the cavity 22 by slot 27.

A thin insulating plate or sheet 33 may be placed over the entire antenna structure, being secured thereto in any appropriate manner as by flange 34 extending outwardly from cylindrical wall 24 in the plane of circular plate 31 and annular disk 26. This insulating plate or sheet 33 serves to seal the interior of the antenna where desirable and also to form a smooth outer surface for the antenna structure so that the entire structure can be mounted flush with and within the outer wall of an aircraft at a desirable location thereof, which is often at the center line of the craft at the bottom of the fuselage thereof.

In antennas for aircraft use, where system eiciency in important, it is desirable for the voltage standing wave ratio (VSWR) value to be less than some moderately low value, as for example 2:1 over the band of frequencies involved, which for example might be from,

225 to 400 megacycles per second. This is generally impossible with an annular slot antenna such as that of Fig. 1 above unless the antenna is made so large that radiation` patterns deteriorate. (As the antenna size increases, for a givenfrequency of operation'the intensity of thesignal toward. the horizon decreases.) In order to keep within this VSWR value, it isnecessary to provide. a matching circuit which in the. present case is afforded invpart by thev annular cavity 28 audits slot 32. In this arrangement the choice between possible alternatives has been made-primarily on practical grounds obtained from experience, and itwill be understood that other` types of matching circuits may be used.

This may be explained byreference to Fig. 2 which shows a schematic diagram of an equivalent circuit of the antenna of Fig. 1.

The impedance of the radiating slot 27 is shown by the series-connected resistor R and capacitor C1 and is designated by the dashed line 27. When using a center feed 11, 12 with a radial transmission line such as 13, 14, the slot reactance'is generally capacitive, which requires that it' be resonated at some frequency in the band by means of some series inductance. This series inductance is indicated at L1 in Fig. 2 and is applied by the cavity 28 and annular slot 32.

In addition, in the present' arrangement a parallel resonant circuit in shunt to line 13, 14 produces two more resonances in the band, and a series transmission line transformer completes the matching. The parallel resonant circuit is provided by the distributed capacitance between flange 16 and the rim 20 of the plate 14 and s shown at C2 in Fig. 2. The inductance L3 of this parallel resonant circuit is supplied by the ring of inductive posts 17. The annular cavity 22 also provides an effective inductance shown at L2 in` Fig. 2 and the transmission line T1 of Fig. 2 may be considered to be the tapered radial line 13, 14.

In designing an antenna of the type shown in Fig. l, the impedance level presented at the input terminals 11, 12 is primarily determined by the width and diameter of the slot 27. In general, if the slot width is decreased, the impedance level is lowered, reducing the value of series inductance required for matching and increasing the required value of shunt susceptance, while decreasing the required characteristic impedance'of the series transformer. It has been found that the Q of the aperture is substantially independent of the slot width for widths which are from l() to percent of the diameter of the annulus, illustratively from 2 to 4 inches, thereby permitting the slot width to be chosen for its effect upon the other elements .inthe antenna.

In general, the lower the aperture impedance selected, the more readily they series inductance can be realized and the' more linear with frequency it will be, because its required reactance will be smaller. However it will then be morediflicult to construct the parallel resonant circuit because it must then have an increased susceptance.

In providing the inductance L1 or L2 connected in series with the aperture, in general two basic ways are available. A short length of transmission line with one end short-circuited can be built into the structure so that its open end is in series with the aperture. This introduces a true reactance in series with the aperture. In accordance with another way, a short section of transmission line Ican be inserted in cascade with the aperture so that the aperture impedance is transformed through it, in which case the impedance change which is introduced is not purely reactive. A resistance decrease of the order of 20% can be expected in using such a transmission line transformer, which is not sufficient to rule out the use of such an element although its effect is undesirable. In the case of a true series inductance an undesirable effect also-occurs in that the input reactance tendsto be a non-linear function of frequency.

In the preferred form shown in Fig. 1, both these methods of deriving series inductance are used. The cavity 28 in one sense may be considered a transmission line with an inner conductor 29 and an outer conductor 25, short-circuited by plate 14 at one end and with its open end 32 in series with the aperture 27. It provides the series inductance L1 of Fig. 2 which is a `true inductance. The cavity bounded by annular disk 26, cylindrical wall 25, plate 14 and dotted line 37, may also be considered a transmission line of annular type shortcircuited at 25 and in series with the aperture 27, and hence is also a series inductance. Similarly, the cavity bounded by flaring wall 15, annular disk 23, the lower part of cylindrical wall 24 and the dotted line 38, may be considered to be a short-circuited length of concentric transmission line providing an inductance in series with the input aperture 27. The remaining portion of cavity 22 forms essentially ashort length of transmission line used as a transformer.

The combination of the inductive effects of all of these portions of cavity 22 is shown by L2 in Fig. 2, and by their use the length of the individual elements is kept short and the overall effect is essentially that of a pure inductance addedin series with the aperture 27 despite the fact that all the inductances are not precisely in series. It will be noted in particular that the input of slot of inductance L1 occurs as an interruption in the front plate itself which produces a small reduction of Q, presumably because the additional annulus raises the resistive component of the aperture impedance. This is a useful concomitant to the desiredl added reactance.

The parallel resonant circuit C2L3 can also be achieved in several ways. Because it is required to be located between the front and back surfaces of the cavity 22, (in order to be contained within the structure) at a constant distance from the aperture (for symmetry), it is distributed along the cylindrical surface inside the antenna cavity structure. In order to avoid short-circuiting the entire cavity 22, it consists physically' of a number of discrete elements symmetrically arranged along this surface. These elements may be individually parallel resonant in the operating band or they may be separate inductances and capacitances appropriately interspersed to provide a distributed effect. In particular, the capacitance lends itself readily to a uniform circumferential distribution and in the example of Fig. 1' is formed in a ring shape with the inductance of the resonant circuit consisting of a series of equally placed posts or wires. While they are indicated as four in number in Fig. 1, any number designed to afford the required inductance may be used. In particular, it has been found that numbers of posts between four and eight are advantageous.

'I'he transmission line which completes the matching of slot 27 to input 11, 12 consists of the radial conical line 13, 14 which is illustrated schematically at T1 in Fig. 2, whose circuit therefore achieves the impedance characteristics of the antenna at theinput terminals l1, i2.

In one illustrative example, suitable dimensions were as follows:

Inches Diameter of disk 31 13 Outer diameter of slot 32 l5 Outer diameter of annular disk 26 19 Inner. diameter of cylindrical wall 24 24 Distance between plate 31 and plate 14 2.178 Overall thickness 4.46

In addition, the insulating plate 33 was made of V14-, inch Fiberglas and ally cavities are filled with a S-pound Lockfoam, which is a sponge-type dielectric materia, being a form of polyisocyanate foam. The effect of the Fiberglas cover is that of an additional capacitance in shunt with the aperture which raises the effettive Q slightly, of the order of 5%. The Lockfoam dielectric has a similareifect upon the effective aperture impedance but also changes the dielectric constant within the cavity, which has the major effect. This Lockfoam dielectric in addition to keeping out moisture adds mechanical strength to the structure if required.

While the structure of Fig. 1 has been illustrated as having a flat outer surface (i. e., the surface containing the annular slots) this is not an essential feature of the invention, and Where necessary to conform to curved surfaces in which the antenna is to be installed Hush, the outer surface may be suitably and comformably curved. This is illustrated in Fig. 3 showing essentially the same structure as in Fig. 1, but with an outer cylindrical surface which, by way of example, may have a radius of curvature of 24 to 48 inches.

In the structure of Fig. 3, all diameters of cylindrical members may be the same as in Fig. l and all circular metal sheets may be of the same diameter as in Fig. 1. The cylindrical walls are kept of the same maximum height along the inner center line and taper down on both sides from that center line to form essentially an intersection between a cylindrical surface and the structure of Fig. 1. In this structure of Fig. 3, the VSWR remained within the limits of 2:1.

Where desired to match the antenna of Figs. 1 or 3 completely to the operating conditions, the inductive portion L3 of the tuned circuit of Fig. 2 may be modilied by changing the lengths of the posts 17. In one form of structure these posts may be made adjustable in length, as by being formed as screw elements threaded into plate 17 and carrying a short-circuiting plunger to form the element 19.

While the embodiments of the invention just described are illustrated as circular in form and with circular slots, these are not essential features lof the invention. The structure may have non-circular contigurations, such as square, rectangular, oval, elliptical etc., and the slots also may have varying configurations, such as square (with or without rounded corners), rectangular, oval, elliptical or other closed-loop forms, all of which are designated herein generally by the term annulan Furthermore, the slots need not be of uniform or constant width; for example, in one form the inner edge ofthe active slot may be circular or generally oval, while its outer edge may be square or rectangular, with or without rounded corners; or the inner edge may be elliptical, and the outer edge elliptical with straight chordal edges at the ends.

It will thus be understood that the foregoing embodiments are illustrative only, the invention being dened by the appended claims.

What is claimed is:

1. A recessed annular-slot antenna comprising an antenna structure having a substantially continuous wall adapted to be mounted flush in a conductive surface, said structure also having means defining a generally annular cavity with said wall and said wall comprising a generally annular conductive membersurrounding and substantially coplanar with a curved-outline conductive plate to define a generally annular slot therebetween, a radial conical transmission line within said structure, a coaxial line connector at the center thereof, means defining a generally annular distributed capicitance at the outer rim of said radial line, said last means comprising means coupling said radial line to said cavity, and symmetrically located inductive means resonating said distributed capacitance.

2. An antenna as in claim 1 further including means Within said structure dening a second generally annular cavity substantially concentric with said first cavity and having a generally annular slot in said wall adjacent said rst slot to provide an inductance electively in series with said rst slot.

3. An antenna as in .claim 1 wherein said capacitancedefining means comprises a flange connected to the outer rim of one conductor of said radial line and adjacent the other conductor thereof, and wherein said inductive means comprises a ring of conductive posts connecting said flange to said other conductor.

4. A generally annular-slot antenna comprising a coaxial line connector, a radial transmission line section symmetrically located relative to and coupled to said connector, means defining a substantially annular cavity surrounding said line section and distributedly coupled thereto, said cavity-dening means having means defining a generally annular slot therein, said cavity-defining means also having means forming a first section thereof as an effective inductance in series between said radial line section and said slot.

5. An antenna as in claim 4 wherein said cavity-defining means also has means forming a second section thereof as an effective inductance in series with said radial line section, and a third section thereof as an impedance transformer between said second section and said slot.

6. An antenna as in claim 5 further including means outside said .cavity for providing a further distributed inductance effectively in series with said slot.

7. An antenna as in claim 6 wherein said last means I comprises means forming a generally annular cavity substantially concentric with said rst cavity and with a second generally annular slot adjacent said rst slot.

8. An antenna as in claim 7 wherein said second slot is coplanar with and substantially equally spaced from said first slot.

9. A closed-loop slot antenna comprising means dening a cavity having a closed-loop slot in a wall thereof, means defining an inductance effectively in a series circuit with the impedance defined by said slot, means defining a resonant circuit in shunt with said series circuit, a uniform characteristic impedance transmission line section having one end coupled to said shunt circuit, and an input connection `at the other end of said line section.

10. An antenna as in claim 9 wherein said inductance defining means comprises means defining an inductance effectively in series with said one end of said line section and means in said ,cavity defining an impedance transformer coupling said latter inductance with the impedance of said slot.

11. An antenna as in claim 10 comprising further means defining an inductance eiectively in series with said slot impedance.

12. A generally annular-slot antenna comprising means defining a generally annular cavity having a substantially planar generally annular slot, a coaxial line connector, means symmetrically coupling said connector to said cavity and including a distributed resonant circuit, and a second parasitic generally annular cavity having a generally annular slot substantially coplanar with and concentn'c to said tirst slot.

13. A closed-loop-slot antenna comprising conductive means defining a cavity having a ,closed-loop slot in one face thereof, means adapted to couple radio apparatus to said cavity, and means defining a second parasitic cavity witha second closed-loop slot adjacent said rst slot.

14. A closed-loop-slot antenna structure comprising means defining a generally annular cavity having a closedloop-slot in a wall thereof, a radial conical transmission line section concentric with said cavity, means at the outer edge of said line section defining an annularly distributed capacitance in the form of a pair of juxtaposed annular conductive surfaces, and a ring of inductive posts interconnecting said surfaces.

No references cited. 

